1. Field of the Invention
The present invention relates to a dual-band (mode) mobile telephone terminal device, and more particularly, to the structures of an attenuator and a switch of a high frequency part disposed inside a radio part of such a mobile telephone terminal device.
2. Description of the Related Art
A digital method (such as PDC) requires that the intensity of an electric wave received by a base station from a mobile telephone terminal device remains constant regardless of a change of a distance between the mobile telephone terminal device and the base station. Hence, a sender part of the mobile telephone terminal device performs gain control.
FIG. 9 schematically shows a positional relationship between a base station and a mobile telephone terminal device. In FIG. 9, one base station BS has a cell range CL which is at a radius of dozens of kilometers, e.g., about 30 km. Within the cell range CL of this base station BS, there are more than one mobile telephone terminal devices TH1 and TH2 which are not at the same distance from the base station BS or which are not under the same telecommunications conditions such as geography. These more than one mobile telephone terminal devices TH1 and TH2, while constantly changing their distances from the base station BS or the telecommunications conditions, simultaneously communicate with the base station BS.
In such a situation, considering the size of the cell range CL, the gain control width in the sender part of the mobile telephone terminal device must be about 50 dB or more so that the intensity of an electric wave received by the base station from the mobile telephone terminal device will be the same between the closest place to the base station BS and the farthest place from the base station BS within the cell range CL of the base station BS. This is known as the near-far problem.
If the sender part of the mobile telephone terminal device fails to achieve excellent gain control, the intensity of an electric wave reaching the base station will mount as the distance between the mobile telephone terminal device and the base station decreases. This will increase leakage power to a neighboring channel. As a result, the digital error rate will grow and the speech quality will deteriorate.
FIG. 10 shows the intensities of receive signals on the respective channels within the base station and an inter-modulation distortion characteristic. In FIG. 10, the solid lines A1 through A6 denote the intensities of electric waves received by the base station on the respective channels and the broken line B4 denotes the inter-modulation distortion characteristic on the channel A4. From FIG. 10, it is seen that the intensities of electric waves received on the channels A3 and A5 are dominated by a distortion component on the channel A4 which is denoted at the broken line B4 and correct data therefore cannot be restored from the channels A3 and A5 which are adjacent to the channel A4.
The sender part of the mobile telephone terminal device preferably performs gain control at its high frequency part, at which the level of a carrier signal is high, as much as possible in order to maintain a state that a ratio (C/N) of the carrier signal level to the noise level is high. This is because the level of the carrier signal is far higher than the level of a background noise at the high frequency part, and therefore, even when the gain is lowered at the high frequency part, the state that the carrier signal is far different from the noise level is maintained.
On the contrary, since the level of the carrier signal is low at an intermediate frequency part, when the gain is lowered at the intermediate frequency part, the difference between the level of the carrier signal and the level of a background noise becomes marginal and thus shrank difference between the carrier signal level and the background noise level will hence be directly passed on to the high frequency part.
For the purpose of gain control over the range of 50 dB or higher, within the sender part of the radio part of the mobile telephone terminal device, the high frequency part controls the gain stepwise and the intermediate frequency part controls the gain continuously. The use of both the gain control at the high frequency part and the gain control at the intermediate frequency part realizes gain control over the range of 50 dB or higher.
Gain control in a mobile telephone terminal device is performed in the following manner. That is, based on the intensity of a receive signal received by the mobile telephone terminal device, the mobile telephone terminal device sets a target value for sending power which is needed to maintain the intensity of the receive signal at the base station constant. The target value is compared with the actual sending power, a feedback control loop which makes the sending power follow the target value is built, and gain control is executed so that the sending power will be the same as the target value.
FIG. 11 shows a base station BS(A) using a band A (mode A) and its cell range CL(A) and a base station BS(B) using a band B (mode B) and its cell range CL(B). Also illustrated in FIG. 11 are mobile telephone terminal devices TH0, TH1, TH2, TH3 and TH4 which are under different telecommunications conditions, that is, which are at different distances from the mobile telephone terminal device TH1.
A dual-band (mode) mobile telephone terminal device, as shown in FIG. 11, switches between the bands (modes) when moving out from the cell range CL(A) of the base station BS(A) to the cell range CL(B) of the base station BS(B). The bands A and B indicate that the terminal device uses different frequency bands, while the modes A and B indicate that the terminal device uses different systems.
While gain control used to ensure that the intensities of electric waves received by the base station BS(A) from the mobile telephone terminal devices TH0, TH1 and TH2 would be the same within the cell CL(A), instantly upon entry into the cell CL(B), the mobile telephone terminal devices TH0, TH3 and TH4 switch the band (mode) and gain control then ensures that the intensities of electric waves reaching the base station BS(B) would be the same.
The structure and operations of a conventional mobile telephone terminal device will now be described with reference to FIG. 12. This mobile telephone terminal device, as shown in FIG. 12, is comprised of a baseband part 100 which is formed by a microcomputer logic part or the like and processes a speech signal and a radio part 200 which receives the speech signal processed by the baseband part 100 and communicates with a base station.
The radio part 200 comprises a sender part 210 which generates a send signal to the base station and a receiver part 220 which receives the send signal from the base station.
The sender part 210 comprises an intermediate frequency part 230, a band-A high frequency part 240 and a band-B high frequency part 250. The intermediate frequency part 230 modulates a speech signal fed from the baseband part 100, adjusts the gain of an intermediate frequency signal and performs mixing for frequency conversion. The high frequency parts 240 and 250 each amplify high frequency signals outputted from the intermediate frequency part 230 and supply to an antenna 300 via a switch 310.
The intermediate frequency part 230 is comprised of a modulator 231 which modulates an intermediate frequency signal in accordance with the speech signal fed from the baseband part 100, a variable gain intermediate frequency amplifier 232 which amplifies, using a variable gain, the intermediate frequency signal which is an output signal from the modulator 231, and a mixer 233 which is for converting an output signal from the variable gain intermediate frequency amplifier 232 into a high frequency.
The variable gain intermediate frequency amplifier 232 described above is formed by a bipolar transistor in many instances. The variable gain intermediate frequency amplifier 232 is capable of varying the gain over the range of about 30 dB at the linearity of ±1 dB. In this case, the gain is continuously controlled over the range of about 30 dB, by means of a gain control voltage which changes continuously.
The high frequency part 240 is comprised of a variable gain high frequency amplifier 241 which amplifies, using a variable gain, the band-A high frequency signal outputted from the intermediate frequency part 230, a power amplifier 242 which power-amplifies an output from the variable gain high frequency amplifier 241 and a switch 245 which is for selecting the band A. The variable gain high frequency amplifier 241 described above is capable of varying the gain over the range of about 20 dB at the linearity of ±1 dB. In this case, the gain is continuously controlled over the range of about 20 dB, by means of a gain control voltage which changes continuously.
The variable gain high frequency amplifier 241 is comprised of a preamplifier (intermediate power amplifier) 243 and an attenuator 244 which is cascaded with the preamplifier 243 and varies the gain of the band-A high frequency signal which is supplied to the power amplifier (high power amplifier) 242. The attenuator 244 is equipped with a function of changing the attenuation over the range of about 20 dB at the linearity of ±1 dB.
The high frequency part 250 is comprised of a variable gain high frequency amplifier 251 which amplifies, using a variable gain, the band-B high frequency signal outputted from the intermediate frequency part 230, a power amplifier 252 which power-amplifies an output from the variable gain high frequency amplifier 251 and a switch 255 which is for selecting the band B. The variable gain high frequency amplifier 251 described above is capable of varying the gain over the range of about 20 dB at the linearity of ±1 dB. In this case, the gain is continuously controlled over the range of about 20 dB, by means of a gain control voltage which changes continuously.
The variable gain high frequency amplifier 251 is comprised of a preamplifier (intermediate power amplifier) 253 and an attenuator 254 which is cascaded with the preamplifier 253 and varies the gain of the band-B high frequency signal which is supplied to the power amplifier (high power amplifier) 252. The attenuator 254 is equipped with a function of changing the attenuation over the range of about 20 dB at the linearity of ±1 dB.
The baseband part 100 includes a control part 110. The control part 110 judges the band for the high frequency signal to be sent based on the receive signal received at the receiver part 220 and adds a switch voltage VSW(A) to the switch 245 while adding a switch voltage VSW(B) to the switch 255, thereby selecting the band for the high frequency signal to be sent.
During a telecommunication in the frequency band of the band A, the control part 110 detects the signal intensity of the receive signal received at the receiver part 220, detects the output level of the power amplifier 242, and sets a target value for the output level of the power amplifier 242 in accordance with the signal intensity of the receive signal. The output level of the power amplifier 242 is compared with the target value for the output level of the power amplifier 242, a gain control voltage VC(A) corresponding to the comparison result is added to the attenuator 244, and a gain control voltage VC(C) corresponding to the comparison result is similarly added to the variable gain intermediate frequency amplifier 232. In this manner, the gain at the attenuator 244 and the gain at the variable gain intermediate frequency amplifier 232 are follow-up controlled so that the output level of the power amplifier 242 will coincide with the target value for the output level of the power amplifier 242.
Meanwhile, during a telecommunication in the frequency band of the band B, the control part 110 detects the signal intensity of the receive signal reaching the receiver part 220, detects the output level of the power amplifier 252, and sets a target value for the output level of the power amplifier 252 in accordance with the signal intensity of the receive signal. The output level of the power amplifier 252 is compared with the target value for the output level of the power amplifier 252, a gain control voltage VC(B) corresponding to the comparison result is added to the attenuator 254, and a gain control voltage VC(C) corresponding to the comparison result is similarly added to the variable gain intermediate frequency amplifier 232. In this manner, the gain at the attenuator 254 and the gain at the variable gain intermediate frequency amplifier 232 are follow-up controlled so that the output level of the power amplifier 252 will coincide with the target value for the output level of the power amplifier 252.
Utilizing both gain control at the variable gain high frequency amplifier 241 or the variable gain high frequency amplifier 251 and gain control at the variable gain intermediate frequency amplifier 232, the mobile telephone terminal device described above realizes gain control over the range of 50 dB or higher.
In accordance with the PDC standard, an input stage of the mixer 233 operates in the 200 MHz band while an output stage of the mixer 233 operates in the 940 or 1441 MHz band. As for the signal levels at the respective parts in such a state that the mobile telephone terminal device yields the maximum output, the signal level at an output terminal of the power amplifier 242 or the power amplifier 252 is +30 dBm (where 0 dBm=1 mW), the signal level at an output terminal of the variable gain high frequency amplifier 241 or the variable gain high frequency amplifier 251 is +8 dBm, the signal level at an output terminal of the switch 245 or the switch 255 is −16 dBm, the signal level at an output terminal of the mixer 233 is −15 dBm, and the signal level at an output terminal of the variable gain intermediate frequency amplifier 232 is −20 dBm.
When the variable gain high frequency amplifier 241 controls the gain over the range of 20 dB and the variable gain intermediate frequency amplifier 232 controls the gain over the range of 30 dB, the signal level at the output terminal of the variable gain intermediate frequency amplifier 232 changes in the range of −20 dBm through −50 dBm. Meanwhile, the signal level at the output terminal of the mixer 233 changes in the range of −15 dBm through −45 dBm. The signal level at the output terminal of the switch 245 or the switch 255 changes in the range of −16 dBm through −46 dBm. The signal level at the output terminal of the variable gain high frequency amplifier 241 or the variable gain high frequency amplifier 251 changes in the range of +8 dBm through −42 dBm. The signal level at the output terminal of the power amplifier 242 or the power amplifier 252 changes in the range of +30 dBm through −20 dBm.
The specific structures of the attenuator 244 (254) and the switch 245 (255) and their operations at the time of switching of the band will now be described with reference to FIGS. 13 through 15.
FIG. 13 is a circuit diagram which shows the structure of the attenuator 244 (254). Such an attenuator 244 (254) controls the gain. The attenuator 244 (254) is comprised of a resistor 2 (12) and a field effective transistor 1 (11) which serves as a series variable resistor, as shown in FIG. 13.
Disposed to the attenuator 244 (254) are a gain control voltage applying terminal 5 (15) which is for applying a gain control voltage VC, a source voltage applying terminal 6 (16) for applying a power source voltage VDD, an input terminal 3 (13) which serves as a signal input part IN for the high frequency signal, and an output terminal 4 (14) which serves as a signal output part OUT for the high frequency signal.
The input terminal 3 described above is connected with the output terminal of the switch 245, while the output terminal 4 is connected with an input terminal of the preamplifier 243. The resistor 2 plays a role of blocking leakage of the high frequency signal. Meanwhile, the input terminal 13 is connected with the output terminal of the switch 255 which is shown in FIG. 12, and the output terminal 14 is connected with an input terminal of the preamplifier 253. The resistor 12 plays a role of blocking leakage of the high frequency signal.
FIG. 14 is a circuit diagram which shows the structure of the switch 245 (255). Such a switch 245 (255) switches the band. The switch 245 (255) is formed by a resistor 22 (32) and a field effective transistor 21 (31) which serves as a series variable resistor, as shown in FIG. 14.
Disposed to the switch 245 (255) are a switch voltage applying terminal 26 (36) which is for applying a switch voltage VSW(A) (VSW(B)), a gate voltage applying terminal 25 (35) which is for applying a ground voltage, namely, a reference voltage GND, an input terminal 23 (33) which serves as a signal input part IN for the high frequency signal, and an output terminal 24 (34) which serves as a signal output part OUT for the high frequency signal.
The input terminal 23 described above is connected with the output terminal of the mixer 233 which is shown in FIG. 12, while the output terminal 14 is connected with an input terminal of the attenuator 244. The resistor 22 plays a role of blocking leakage of the high frequency signal. Meanwhile, the input terminal 33 is connected with the output terminal of the mixer 233 which is shown in FIG. 12, and the output terminal 34 is connected with an input terminal of the attenuator 254. The resistor 32 plays a role of blocking leakage of the high frequency signal.
FIG. 15 is a drawing which shows voltage control characteristics of the attenuator 244, the attenuator 254, the switch 245 and the switch 255 relative to a position to which the mobile telephone terminal device has moved. In FIG. 15, the gain control voltages VC(A) and VC(B) and the switch voltages VSW(A) and VSW(B) are shown.
Operations of the attenuators 244 and 254 and the switches 245 and 255 having such structures described above will now be described. The mobile telephone terminal device is driven at a voltage of up to about 3.0 V by a lithium battery or the like. The threshold voltages of the field effective transistors respectively represent a bias at which the variable resistors initiate the gain control operation and a bias at which the switches initiate the switching operation. The ground voltage (reference voltage) is applied to the gate voltage applying terminal 25 (35) of the field effective transistor 21 (31).
In the cell CL(A) which uses the band A, L (0 V) is applied to the switch voltage applying terminal 26 of the switch 245 to thereby select the band A, while H (3.0 V) is applied to the switch voltage applying terminal 36 of the switch 255 as the band B is not selected. Since the distance from the base station BS(A) is the shortest within the cell CL(A) when the mobile telephone terminal device is located at the spot denoted at TH1, the minimum value (0.5 V) is applied as the gain control voltage VC(A) to the gain control voltage applying terminal 5 so that the attenuation at the attenuator 244 will become maximum.
In this case, when the mobile telephone terminal device moves from the spot denoted at TH1 to the spot denoted at TH0, the gain control voltage VC(A) applied to the gain control voltage applying terminal 5 sequentially changes from the minimum value (0.5 V) to the maximum value (2.5 V) to thereby ensure that the attenuation at the attenuator 244 will change from the maximum to the minimum.
Simultaneously with the arrival of the mobile telephone terminal device at the spot denoted at TH0, within the cell CL(B) which uses the band B, L (0 V) is applied to the switch voltage applying terminal 36 of the switch 255 to thereby select the band B and H (3.0 V) is applied to the switch voltage applying terminal 26 of the switch 245 as the band A is not selected. In this situation, since a distance between the mobile telephone terminal device (TH0) and the base station BS(B) within the cell CL(B) is the longest, the maximum value (2.5 V) is applied as the gain control voltage VC(B) to the gain control voltage applying terminal 15 so that the attenuation at the attenuator 254 will become minimum.
Further, when the mobile telephone terminal device moves from the spot denoted at TH0 to the spot denoted at TH4, the gain control voltage VC(B) applied to the gain control voltage applying terminal 15 sequentially changes from the maximum value (2.5 V) to the minimum value (0.5 V) to thereby ensure that the attenuation at the attenuator 254 will change from the minimum to the maximum.
Meanwhile, in the variable gain intermediate frequency amplifier 232, the gain control voltage VC(C) is changed independently of selection of either the band A or B, whereby the output level changes continuously.
FIG. 16 is a timing chart which shows the timing at which the attenuators shown in FIG. 13 and the switches shown in FIG. 14 operate in response to the switching of the band in accordance with moving between the cells. Shown in FIG. 16 are changes of the level POUT (SW(B)) of the output signal from the switch 255 and the level POUT (ATT(B)) of the output signal from the attenuator 254.
Problems with the conventional technique will now be described with reference to FIG. 16. When switching from the band A to the band B is attained by means of a combination of the switches 245 and 255 and the attenuators 244 and 254, it takes scores of microseconds until the switch 255 has stably turned on since application of the switch voltage for instance as shown in FIG. 16 at the time of the switching of the band, and therefore, the gain control voltage VC(B) is applied to the gain control voltage applying terminal 15 of the attenuator 254 with a delay which is equivalent to this transient response time of the switch 255. Hence, differences of the characteristics of the attenuator 254, the variable gain intermediate frequency amplifier 232 and the like could give rise to a variation in desired gain of the mobile telephone terminal device immediately after the switching of the band.
FIG. 17 is a drawing for describing the problem above with the conventional technique. FIG. 17 shows changes of the level POUT of the output signal from the antenna in a situation that the mobile telephone terminal device is communicating under an ideal condition while moving away from the base station at a constant speed.
In relation to the situation described with reference to FIG. 16, such a situation will now be considered that the mobile telephone terminal device is communicating under an ideal condition while moving away from the base station at a constant speed.
Within the cell CL(B), owing to the gain control function, the level POUT of the output signal from the mobile telephone terminal device is normally supposed to decrease linearly. However, in the situation as described above, as shown in FIG. 17, the delayed follow-up operation attributed to a delay of feedback control and the discontinuity of the output level occurring at the time of the switching of the band temporarily pushes out the level POUT of the output signal of the mobile telephone terminal device off from the linear line at the time of the switching of the band. As this happens, the intensity of the receive signal at the base station deviates from the specified value, a difference develops between the levels of the receive signals on the adjacent channels, and the speech therefore gets disturbed or the speech quality deteriorates.
Although the foregoing has described this problem in relation to an example that the mobile telephone terminal device is moving under an ideal condition, actual conditions of moving are much worse, including such a situation that the intensity of the receive signal abruptly drops low as the mobile telephone terminal device gets behind a building. The problem that the intensity of the receive signal at the base station deviates from the specified value is thus believed to be rampant, adding to the difficulty of the deteriorated speech quality.
In addition, the control part 110 of the baseband part 100 needs be set up with the three types of the gain control voltages VC(A), VC(B) and VC(C) to control the variable gain high frequency amplifier 241, the variable gain high frequency amplifier 251 and the variable gain intermediate frequency amplifier 232, and further, with the two types of the switch voltages VSW(A) and VSW(B), which demands complicated control for the control part 110.
Further, since the high frequency part 240 and the high frequency part 250 respectively require the switch 245 and the switch 255, the circuitry structure is complex and a large space is necessary. This leads to another problem that the mobile telephone terminal device as a whole becomes large.